With an explosive increase in demand of a broadband multimedia communication service such as the Internet or a high-definition digital TV broadcast, a dense wavelength-division multiplexing optical fiber communication system, which is, suitable for a long-distance and large-capacity transmission and is highly reliable, has been introduced in trunk networks and metro networks. In access networks, an optical fiber access service spreads rapidly. In such an optical fiber communication system, cost reduction for laying optical fibers as optical transmission lines and improvement of spectral efficiency per optical fiber are required. Therefore, a wavelength-division multiplexing technology which multiplexes multiple optical signals having different wavelengths is widely used.
In an optical transmitter for such a high-capacity wavelength-division multiplexing communication system, an external optical modulator is a key component. In the optical modulator, high speed operation with small wavelength dependence is indispensable. Further, unwanted optical phase modulation (or wavelength chirping) which degrades the waveform of the received optical signal after long-distance transmission should be suppressed as small as possible. A Mach-Zehnder (MZ) optical intensity modulator in which a couple of waveguide type optical phase modulators are embedded into an MZ interferometer is suitable for such a use.
In general, a currently used MZ optical intensity modulator has a couple of waveguide-type optical phase modulators having two optical paths between optical waveguide type multiplexer/demultiplexer of the MZ interferometer monolithically integrated on a lithium niobate (LN: LiNbO3) substrate which is a typical electro-optic crystal. Further, electrodes for applying the electric field to the waveguide-type optical phase modulator are provided in the vicinity thereof. The LN-based MZ optical intensity modulator modules have some problems with the size (electrode length: about 5 cm, module length: about 15 cm), the driving voltage (about 5 Vp-p), and the like. However, since there is no practical external optical modulator which surpasses the LN-based MZ optical intensity modulator in high-speed long distance optical transmission properties, it is still widely used for an optical transmitter unit or the like in various optical communication systems.
In high speed optical modulation by using this external optical modulator, especially in the high-frequency region in which the frequency of the modulation RF signal is over 1 GHz, the propagating wavelength of the modulation RF signal becomes not negligibly short compared with the electrode length of the optical phase modulator region (the interaction length between the modulated optical signal and the modulation RF signal) in the LN-based MZ optical intensity modulator. Therefore, distribution of the RF signal voltage is no longer regarded as uniform in a longitudinal direction of the electrode which is means for applying electric field to the optical phase modulator. To estimate optical modulation performance exactly, it is required to treat the electrode as a transmission line and treat the modulation RF signal propagating through the electrode as a traveling-wave, respectively. In that case, in order to increase the effective interaction length with the modulated optical signal and the modulation RF signal which are propagating in the optical phase modulator region for improving modulation bandwidth and driving voltage, a so-called traveling-wave type electrode which is devised to make phase velocities vo, vm as close to each other as possible (phase velocity matching) is required.
In order to realize such optical waveguide type modulators, a III-V compound semiconductor such as gallium arsenide (GaAs) or indium phosphide (InP) can also be used so as to apply materials having a (complex) refractive index which changes as applied electric field changes to an undoped optical waveguide core layer. In the case of such semiconductor-based optical modulators, a semiconductor optical waveguide with the p-i-n structure in which an undoped optical waveguide core layer is sandwiched between a p-type cladding layer and an n-type cladding layer is often used so as to apply the electric field to the core layer by applying reverse bias voltage.
In 1550 nm band mainly used in the optical fiber communication system, a single mode optical waveguide with the p-i-n structure is suitable for practical use. In the case of utilizing the optical waveguide with the p-i-n structure as the transmission line of the modulation RF signal for the traveling-wave type optical modulators, the p-type semiconductor which usually has lower electrical conductivity as a clad layer than the n-type semiconductor decreases the (complex) characteristic impedance (absolute value) of the transmission line to about 20Ω, which is less than half of the typical characteristic impedance (50Ω) of widely-used microwave circuit components. This impedance mismatch leads to degradation of the modulation bandwidth due to reflection or the like and increase in the power consumption of the driving circuit when the modulation RF signal output from the driving circuit is applied to the optical modulator as a transmission line.
For the same reason, the effective refractive index nm (=c0/|vm|, c0: velocity of light in free space) which affects the modulation RF signal is about seven on average. This value is about twice the effective refractive index no (=c0/|vo|, about 3.5 in InP) of the optical signal which propagates along the waveguide. This phase velocity mismatch between the modulated optical signal and the modulation RF signal leads to decrease in the effective interaction length between both. Further, there are some problems about the modulation bandwidth or the driving voltage like in the case of the above-mentioned impedance mismatch.
Further, the p-type semiconductor has a larger optical-absorption coefficient than that of the n-type semiconductor. Therefore, when using it as a cladding layer of the long optical waveguide in the traveling-wave type optical modulator, the attenuation of the modulated optical signal due to the absorption in the p-type cladding layer often leads to a higher insertion loss.
In this way, the p-i-n diode structure has some problems as the transmission line of the traveling-wave type optical modulators in achieving lower operating voltage and wider bandwidth.
An n-i-n-type layered structure acquired by replacing the p-type semiconductor cladding layer which is the origin of these problems (of the p-i-n structure) with an n-type semiconductor cladding layer having higher electrical conductivity is considered to be effective for suppressing the above-mentioned impedance mismatch and velocity mismatch. Therefore, smaller driving voltage swing and wider bandwidth by elongating the device size are expected to be compatible. Besides, suppression of the absorption due to the p-type dopant enables lower insertion loss. These features are suitable for the traveling-wave type electrode which has an advantage in realizing high speed and low voltage operation in a semiconductor optical waveguide based optical modulator such as an electroabsorption type optical modulator, or an optical phase modulator.
In the case of the n-i-n layered structure, however, the electric field cannot be applied to the undoped layer because the structure cannot prevent carrier injection to the undoped layer. From the viewpoint of preventing this carrier injection, an n-SI-i-n-type layered structure is considered to be a candidate. In the n-SI-i-n-type structure, a semi-insulating (SI) semiconductor layer doped with impurities having the electron trapping ability is inserted between the undoped optical waveguide core layer and the n-type cladding layer.
As an example for applying this n-SI-i-n-type layered structure, an InP-based semiconductor MZ optical intensity modulator is reported in Non-Patent Document 1. A pair of waveguide type optical phase modulator regions composing this semiconductor MZ optical intensity modulator have the n-SI-i-n-type layered structure composed of an n-InP upper cladding layer, an SI—InP layer, a 0.3 μm thick undoped InGaAlAs/InAlAs multiple quantum well core layer in which transition wavelength between each first quantum level of electron and heavy hole is 1370 nm, and an n-InP lower cladding layer. This waveguide is shaped to be a 2 μm wide mesa stripe by using dry etching technology, and both sides of the mesa stripe are buried with a SiN film and a low-permittivity polymer (benzo-cyclo-butene: BCB). It is a so-called high-mesa ridge structure. By the way, Patent Document 1 can be cited as the related art of the present invention.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-099387.    [Non-Patent Document 1] Kikuchi Nobuhiro and five other persons, “Low Driving Voltage 40 Gbit/s Semiconductor-based Mach-Zehnder Modulator”, Technical report of IEICE, LQE, Nov. 2005, pp. 41-44.